DESCRIPTION: (Adapted from the applicant's abstract.) One of the major functions of the striate cortex is to bring together the information carried by separate populations of neurons in the lateral geniculate nucleus (LGN). Neurons in the LGN are monocular and respond selectivity to either ran increase in the level of illumination that falls on their receptive field centers (ON-center cells) or a decrease (OFF-center cells). In contrast, most neurons in the striate cortex are binocular and show both ON and OFF responses. While the mixing of these inputs is a well established fact, the intracortical circuitry that underlies these changes remains largely unknown. The goal of this research plan is to increase our understanding of the functional organization of striate cortex by examining some of the intracortical circuits that are responsible for ON/OFF and binocular convergence. The input-output organization of layer IV in the tree shrew striate cortex provides a unique opportunity to gain insights into this problem. With funds from our current NIH award we found evidence for two parallel streams that convey ON and OFF information to the superficial layers and we think these systems differ in the way they integrate ON and OFF information from the two eyes. The goal of this application is to test this hypothesis using combination of anatomical and physiological methods. Four specific aims are proposed: 1. To examine the response properties of neurons in different tiers of layer IV; 2. To determine the morphology and the response properties of individual layers IV neurons that project to two tiers within layer III (layers IIIa-b and IIIc); 3. To examine the response properties of neurons in layers IIIa-b and IIIc; 4. To determine whether layers IIIa-b and IIIc project to separate regions of extrastriate cortex. These experiments should provide valuable insights into the functional organization of striate cortex by offering an effective method for relating receptive field transformations to cortical circuitry.